Alloy steel and structural member



Patented Mar. 14, 1939 UNITED STATES PATENT OFFICE No Drawing. Application May 5, 1938,

Serial No. 206,191

Claims.

- In the design of engineering structures it has been recognized that the essential physical prop- 1 5 erty of the steel on which the design is based is the yield point. For most structures a high yield point is desired and, therefore, alloy steels have been generally employed having high yield points. For certain requirements of engineering structures, however, it is desirable that the members have a high tensile strength combined with a comparatively low yield point. Plain carbon steels inherently possess a low ratio of yield point to tensile strength but most alloy steels have a comparatively high yield point for the same tensile strength as compared to carbon steels. If, therefore, an engineering structure requires in the steel the property of a low ratio of yield point to tensile strength, or in other words a largespread between the yield point and tensile strength, most alloy steels, and especially those containing high phosphorus, are precluded from such applications and therefore some of the desirable properties of alloy steels, such as fine grain structure, ductility in the as rolled condition, and especially corrosion resistance derived from the cheap element phosphorus, cannot be taken advantage of in such construction. Among the alloying elements it is well known that phosphorus is the most potent element from the standpoint of raising the yield point. Silicon and copper are also very effective in increasing the yield point. As the yield point, from 'the designing engineer's standpoint,is one of the most important properties of steel, it is evident that any means whereby in an alloy steel the yield point may be varied at will while the other properties, physical and corrosion resistive, can be maintained practically the same, is of great engineering value.

Most composite engineering structures are commonly made of rolled structural members. It is desirable, particularly from the physical and the corrosion resistant standpoints, that the structural members have approximately the same physical properties, such as high tensile strength, ductility, weldabiiity and corrosion resistance, and that they also have approximatelythe same composition since this reduces the danger of electrolytic potential differences. However, in many engineering structures a high yield point is desired in some members and a low yield point in other members, and for such structures it is of great advantage to secure a difference in yield point between different members while maintaining the other characteristics substantially the same.

I have found that in certain types of alloy steels it is possible to control the yield point without material alteration of the other-prop- 5 erties, such as tensile strength, ductility, weldability and main alloying constituents. I have found that by the addition of the proper percentage of molybdenum to an alloy steel of'low carbon content and containing the alloying elements of silicon, copper and phosphorus in amounts properly balanced to each other; the yield point may be reduced without affecting to .any marked degree the physical properties and corrosion resistance properties of such alloy steel. I further found that the reduction of the yieldpoint may be regulated by the amount of molybdenum employed within certain definite limits.

This effect of molybdenum in these particular steels is entirely novel and contrary to its effect an in the usual steels. Molybdenum in the usual steels behaves like other strength imparting elements, increasing both the tensile strength and the yield point, the yield point even in a greater measure than the tensile strength. So far as I am aware, it is only when molybdenum is added to the herein described alloy steel compositions that a it produces this novel and anomalous property of decreasing the'yield point without materially affecting the tensile strength.

The steels which I have discovered may have their yield point lowered by the addition of molybdenum are low carbon alloy steels containing carbon from .01 .to .35%, silicon from .35 to 2.0%, copper from .18 to 1.15% and.phosphorus from .08 to 35% as the essential alloying constituents.

I have found that steels of the following ranges of composition have a very good combination of physical. properties, together with fair corrosion resistance, making them particularly useful for 40 special requirements: carbon .03 to 20%, silicon .50 to 1.5%, copper .25 to 1.0% and phosphorus .10 to 22%. The preferred ranges of the carbon and alloying elements are carbon .05 to .15%, silicon .60 to .90%, copper .30 to .50% and phosphorus .10 to .16%. I have found that the yield point of such steels is materially reduced without substantial reduction of the tensile strength, by the addition of molybdenum in an amount from .15 to .65%, preferably from .17 to .25%. I have found that additions of molybdenum in amounts less than about .-15% to steels of this character do no efiect a material reduction in the yield point, and by material reduction, I mean a reduction in the order of 5000 pounds or more per square 1!;

- such as nickel and chromium, may be employed in addition to the silicon, copper and phosphorus to impart additional corrosion resistance, the bal ance of the composition will always be principally iron, at least and preferably at least 97% iron.

When molybdenum is added to steels of the above described composition, its novel effect is probably due to the formation of a new constituent in the structure of the steel, probably some combination of molybdenum and the phosphorus, or of molybdenum and phosphorus together with some of the copper. This constituent lowers the yield point characteristics but does not adversely affect the tensile or corrosion resistive properties of the steel. The formation of such constituent apparently takes place only when a definite appreciable quantity of molybdenum, namely, above .15%, is present. The quantity of molybdenum added to offset the high yield point essentially due to phosphorus together with copper and silicon bears a definite relationship to the amount of phosphorus present in the steel. I have found that the percentage of molybdenum present must be at least equal to the percentage of phosphorus present, and that as the molybdenum is increased beyond this quantity, further depression of the,

yield point takes place proportional to the extra molybdenum added within certain limits.

Steels containing the elements silicon, copper and phosphorus, treated by the molybdenum addition, show a fair degree of corrosion resistance, superior to ordinary copper-bearing steels. I have found, however, that I can add small amounts of chromium and/or nickel, preferably chromium, to the above compositions in order to further augment their corrosion resistance without sacrificing the essential feature of my invention, namely, the reduction of the yield point by the action of molybdenum. For imparting this additional corrosion resistance, I add chromium or nickel, either singly or in combination. The amount of chromium employed is from .25 to less than 2.0%, preferably from .50 to 1.25%. The amount of nickel employed is from .25 to less than 2.0%, preferably from .50 to 1.25%. When ance to mild corroding media, such as the atmosphere, municipal and mine waters, sea water and the like, high tensile strength combined with ductility and weldability and a yield point which is materially lower than that of the steel containing no molybdenum but otherwise of the same composition: carbon .03 to 20%, silicon .50 to 1.5%, copper .25 to 1.0%, phosphorus .10 to .22%, chromium .5 to 1.5% and molybdenum 15 to 35%. The following is a typical more specific analysis composition of a steel of this type: carbon .05 to .15%, silicon .60 to .90%, copper .30 to .50%, phosphorus .10 to 6%. Ch o to 1.25% and molybdenum .17 to .25%.

In these last mentioned compositions which contain chromium, the balance of the composition is substantially all iron except for impurities and the usual minor amounts of other elements.

usually present in steels of this type.

It seems to be an inherent property of the combination of silicon, phosphorus and copper to permit the action of molybdenum in lowering the yield point as the presence of other elements, either of the ferrite strengthening type or of the carbide forming type such as nickel and chromium Within the ranges here indicated, does not interfere with this action of molybdenum. A ferrite strengthening element like nickel in the above-specified amounts might be expected to be a neutral element not interfering with the action of molybdenum but it is entirely unexpected to find that a strongly carbide forming element, chromium, one which exerts a marked influence on other carbide forming elements, should also not interfere with the effect of the molybdenum in reducing the yield point. The addition of chromium in alloy steels is known to raise both the tensile strength and the yield point. This action is'especially apparent when the chromium is present together with molybdenum. Therefore, the neutral behavior of chromium towards molybdenum in the presence of the combination of copper, silicon and phosphorus above specifiecl, is especially remarkable. The combination of copper, silicon and phosphorus, therefore, functions in permitting the molybdenum to lower the yield point irrespective of the presence of nickel and chromium in the above-specified amounts.

The following table shows typical analyses of steels of the type above mentioned both with and without molybdenum, together with determinations of the tensile strength, yield point, elongathe chromium and nickel are used in combmation and reduct1on in area:

Red.

4 Elong. Tensile Yield point area Steel No. C Si P Mn Cu Cr N1 M0 lbsJsqm mSJsqJn' perc ent pep cent tion, the total amount of the two should not exceed 3.0% and both constituents should be less han each. Steels of the following ranges of composition have excellent corrosion resist- As indicated by this table the presence of mo- 'lybdenum within the ranges indicated above effects a material and in most cases a large reduction of the yield point over that of a steel of similar composition but containing no molybdenum.

' Also as indicated by the table, the other alloying indicated without interfering with the peculiar.

property of the molybdenum of lowering the yield point in steels of this character.

In engineering structures where strains are set up during the processes of fabrication, a lower yield point steel with retainment of the other properties imparted by the alloying elements, such as tensile strength, ductility, impact resistance, corrosion resistance, etc., offers great advantages. Strains are especially set up in structures which are produced by welding and, therefore, in welded structures the value of maintenance of all properties and controlling the yield point becomes especially pronounced. Control of the yield point helps to obviate the necessity of heat treatment of the structure after welding to relieve welding strains, which is a decided advantage. A lower yield point isalso very desirable when the steel is subjected to deep drawing operations.

Thesesteels are preferably formed in rolled structural members, such as shapes, plates, sheets, etc., which may be employed in various engineering constructions, such as bridges, buildings, railroad cars, trucks, ships, etc. where there is desired a high tensile strength combined with ductility, corrosion resistance and weldability but with a yield point materially lower than that of alloy steels of corresponding tensile strength. If desired, composite structures may be made in which the members having high yield point may be formed of the steel without molybdenum and l the members requiring a low yield point may be The other usually occurring elements commonly found in all steels, such as sulphur, manganese and the like, may be presentto the usual extent, although I prefer to have the contents of such elements as low as possible, for instance, that of manganese below .6% and of sulphur below .05%. The steel in the process of manufacture can be treated with the usual refining agents such as aluminum, titanium, vanadium, zirconium and the like, and the presence of these elements to the extent as found in all steels thus treated does not adversely afiect the property of controlling the yield point by the addition of molybdenum.

My steels may be manufactured by the ordinary processes of steel making, such as the open hearth, Bessemer or electric furnace. The steel may be forged, rolled or fabricated, according to standard practice, or it may be poured into molds for the production of castings. These steels display the desired properties in the as rolled condition, but they may be heat treated suitably to alter any of the properties by the usual annealing, normalizing, quenching or'other heat treating processes.

The alloying elements silicon, copper, phospho rus, chromium, nickel and molybdenum may be introduced into the steel according to the usual alloying practice. The copper, nickel and molybdenum may be added in the furnace, as these elements do not become oxidized during the process of steel melting. However, slight amounts of them may be added in the ladle to adjust the final analysis of the steel. The silicon may be added in the form of commercial ferro-silicon alloys of various grades in the furnace, although some silicon in the form of a high percentage, 75 or grade, may be preferably added in the ladle to adjust the final analysis. Phosphorus is added. in the form of term-phosphorus and while it can be added in the furnace, on account of its rapid oxidizability it is recommended to addgthe major bulk, if not all, in the ladle after tapping the steel. The chromium, usually in the form of ferro-chrome, may be added to the furnace just before tapping the steel, and further, additions made in the ladle to adjust the final analysis.

While I have specifically described the preferred embodiment of my invention, the invention may be otherwise embodied and practiced within the scope of the following claims.

I claim:

1. Rolled structural members having high tensile strength combined with ductility, corrosion resistance and weldability and requiring a ratio of yield point to tensile strength lower than that which is characteristic of the usual allo-y steels, composed of .an alloy steel of low carbon content between .01 and .35% carbon and containing as essential alloying constituents .35 to 2.0% silicon, .18 to 1.15% copper, .08 to 35% phosphorus, and .15 to .65% molybdenum, the molybdenum content being not less than the phosphorus content, the balance being at least iron, and characterized by a'yield point materially lower than that of a steel containing no molybdenum but otherwise of the same composition.

I, 2. Rolled structural members having high tenbetween .01 and .35% carbon and containing as essential alloying'constituents .35 to 2.0% silicon,

.18 to 1.15% copper, .08 to .35% phosphorus, .25 to less than 2.0% chromium and .15 to .65%

\ molybdenum, the molybdenum contentbeing not less than the phosphorus content, the balance being substantially all iron, and characterized by yield point materially lower than that of a steel containing no molybdenum but otherwise of the same composition.

3. An alloy steel of low carbon content between .01 and .35%carbon and containing as essential alloying constituents .35 to 2.0% silicon, .18 to 1.15% copper, .08 to .35% phosphorus, and .15 to .65% molybdenum, the molybdenum content being not less than the phosphorus content, the balance being at least 95% iron, and characterized by high tensile strength combined with ductility, corrosion resistance and, weldability, and by a yield point materially lower than that of a steel containing no molybdenum but otherwise of the same composition.

' 4. An alloy steel of. low carbon contentbetween .01 and .35% carbon and containing as essential alloying constituents .35 to 2.0% silicon, .18 to 1.15% copper, .08 to .35% phosphorus, .25 to less than 2.0% of an element of the group consisting of nickel and chromium, and .15 to .65% molybdenum, the molybdenum content being not less than the phosphorus content the balance being at least 95% iron, and characterized by high tensile strength combined with ductility, corrosion resistance and weldability, and by a yield point materially lower than that of a steel conthan the phosphorus content, the balance being substantially all iron, and characterized by high tensile strength combined with ductility, corrosion resistance and weldability, and by a yield point materially lower than that of a steel containing no molybdenum but otherwise of the same composition.

BYRAMJI D. SAKLATWALLA. 

